Extract particle age info as render layer
You can do this by adding a new render layer with an Override Material specified.

First create a dummy object and give it a new material and call it something like particle_age.  Just plug the Particle Age output into an Emission shader and plug that into the output.  Also make sure to give it a Fake User.



Then just add another render layer and give it the new material as an Override Material.  You can also override the samples with a small amount, since emission shaders are easy to compute.